Procedure and sliding support for a profilometer

ABSTRACT

A movable support (5) provided at its botton surface with a pad (19) made of antifriction material and slidable on a lapped reference surface (2). 
     The movable support includes discharging jacks (8) which bear indirectly against the frame. The effort corresponding to the sum of the independent efforts of each jack must be lower than the effort exerted by the weight of the movable support and of its optional accessories, so as to minimize the difference between the sliding effort at the beginning and the friction effort of displacement in any direction on one plane and so as to limit, in the displacement referential X,Y,Z, the deviations of the geometrical positions of a sensor or of a tool. A motor (26) and a particular reducer drive the movable support in a translation direction X. There is provided a second drive unit for a translation in the direction Y. 
     By using as an accessory a micrometric displacement table and a follower, it is possible to take tridimensional profilometric measurements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention deals with a procedure and a device for displacement bysliding a mobile support on which accessories such as a part, a sensoror a tool can be fitted over a reference surface, such that distributedforces can be applied to this mobile support from the bottom upwards.

2. Description of the Prior Art

It is known that every displacement of an object in space is subjectedto three geometric deviations from its movement yaw, pitch and roll.

Known solutions, using earlier technologies, propose the use of twoslides superimposed on intersecting movements which provoke thesuperposition of these three geometric deviations and theirinteractions; moreover, this type of device is tributary to themachining tolerances and to the respective vibrations of each slidewhich propagate through the sensing system, leading to the existence ofdefects which can only partly be eliminated. However, in order toimprove the performances of existing devices, large and heavy systemshave been designed in order to limit the adverse effects of vibrationsby the inertia of the device itself. It can be seen that these largeparts also have to be accurately machined: leading to equipment which isdisproportionately expensive compared with the results obtained.

Another known solution consists of displacing the mobile part relativeto a surface, without it coming into contact with it, by means of anair-cushion system. In this case too, vibrations induced by thesecontrolled air leakage systems make this solution unsuitable for somespecial applications. The air film always has some elasticity and formsa vibration amplifier.

Moreover, the most obvious solution consists of displacing the mobilepart by sliding over a reference surface, however the contact pressureis proportional to the mass, thus eliminating all reproducibility ofresults. In order to be moved by sliding, any mobile part must be pulledor pushed by a force greater than that necessary to maintain it infriction displacement.

The difference in the sliding and friction forces applied to the mobilepart controls its geometric position within a coordinate system.Moreover, these frictional values provoke dimensional deviations due totemperature variations; dissymmetry of the load distribution is anotherdirect consequence which is observed and which affects the displacementquality, especially during stopping and starting of a mobile part.Finally there is a premature wear phenomenon of sliding surfaces incontact in this type of construction. Moreover, in order to displace themobile part, it has to be subjected to as uniform movements as possible,which does not arise in practice.

Electric motor displacement systems with a shaft driving a rotating boltin a ball screw introduce new defects in addition to the initialdefects, which when accumulated make it impossible to carry out highprecision measurements. For some applications, for example surfacecondition measurements, the dispersion of measurements on a profiletrajectory can reach a value of 5 micrometers in Z, regardless of thetype of sensor placed on the supporting mobile part.

For example, we can quote the case of mean values of irregularityspacings for different types of machining, and spacing (e) and depth (p)values in micrometers:

    ______________________________________                                                   spacing  depth                                                     ______________________________________                                        Lathe work   280 < e < 60                                                                             1.5 < p < 15                                          Grinding     180 < e < 7                                                                                4 < p < 0.5                                         Milling      400 < e < 17                                                                             3 < p < 4                                             ______________________________________                                    

It will be noted that these values require high measuring precision,especially among the Z-axis. At the present time, displacement systemsusing two intersecting movement tables driven by stepping motors givemeasuring accuracies of the order of 5 micrometers, incompatible withtechnical surface irregularity depth measurements. In addition, standardISO TC 57 uses the term micro-roughness when the depth of irregularitiesis less than 0,5 micrometers.

There is already a measurement device which uses a procedurecorresponding to the foreword in claim 1 (United States patent U.S. Pat.No. 3,377,711). The mobile support displaces along rails by means ofroller bearings and is laterally supported by jacks, on which the end ofthe rod is fitted with a rolling device and for which the offset islimited by roller end stops built into the support and which bear underthe rails. This type of procedure does not give a high precisiondisplacement relative to a reference surface, but simply a temporaryremoval of pressure on the roller bearings during support positionchanges, in order to avoid deformation of the guide rails which form thegeometric displacement reference.

SUMMARY OF THE INVENTION

The purpose of this invention is to correct these disadvantages. Thisinvention, as distinguished in claim 1, solves the problem ofmanufacturing a device facilitating displacement of a support fittedwith an accessory by sliding across a plane reference surface, byreducing the pressure between the said support and the said referencesurface to a value which is just sufficient to maintain contact, inorder to minimize the difference between the sliding and friction forcesto minimize geometric deviations of position in the displacementcoordinate system, in other words the uncertainty of displacementsensors in X or in Y, or in X and Y simultaneously.

The purpose of the invention procedure is to displace a mobile supportby sliding across a plane X-Y reference surface by means of two motors,and this mobile support can be fitted with accessories, a part, asensor, or a tool. This procedure is distinguished mainly in thatdistributed forces are applied to the mobile support from the bottomtowards the top, such that the sum of these forces is always less thanthe weight of the support and the accessories. The procedure requiresthat sliding is modified to suit the operation carried out byaccessories along the Z-axis. The procedure is also distinguished inthat the geometric depth deviations of a part are measured continuouslyand by displacement of the mobile support, using an optical reading,capacitive or thermal fluid flow dimensional measurement sensor mountedon a micrometric forward motion table rigidly attached to this mobilesupport.

The procedure is also distinguished in that it requires thatdisplacement of the mobile support by sliding takes place during themeasurement within the measurement range of the sensor and isinterrupted when the limit of the range is reached, while themicrometric forward motion table translates the sensor along the Z-axisin order to start a new continuous measurement of geometric deviations.

The device making use of this procedure includes a mobile support whichis fitted with at least three jacks which, through roller devices orfriction skids on the upper face of the slides and a skid attached toits lower face, made of a material with a low coefficient of friction,self-lubricating, wear-resistant, and accurately machinable; the skidbears on the reference surface. This mobile support is displaced bymeans of a screw coupled to the output spindle of a reduction gearthrough a joint which is fixed in torsion and in tension and is free inall other degrees. The reduction gear is the "Harmonic Drive", driven bya stepping motor. The screw is mounted in a four degree of freedomself-aligning type nut, integral with the mobile support. It is alsodistinguished in that an accessory such as a stepping motor drivenmicrometric forward motion table is rigidly mounted on the mobilesupport. The displacement direction of this table is perpendicular tothe reference surface, in other words along the Z-axis.

It is obvious that this type of procedure and device has manyadvantages, including the following:

a low load on the sliding surfaces, with the advantage of reducing thestarting force to make it approach the value of the sliding force, and amore accurately positioned stop and, consequently, and minimizinggeometric positional deviations of the sensor when the table starts andstops, leading to a better response.

less heating of parts in motion, and therefore lower thermal geometricdeformations, resulting in a better displacement quality.

a reduction of the rotation torque applied to the screws, and thereforeminimizing positional uncertainty of elements, at the same time as usinglower capacity motors.

high guidance accuracy

the possibility of measuring geometric profile deviations such as:deviation of shape, rippling, roughness and, by X-Y displacement,surface level curves.

BRIEF DESCRIPTION OF THE DRAWING

Other advantages and characteristics will result from reading thespecial non-restrictive methods of manufacture, referring to theattached figures which represent:

FIG. 1: a perspective view of a device making use of the invention;

FIG. 2: the installation on the mobile support of a micrometricdisplacement table for measuring the geometric surface condition;

FIG. 3: a lateral view of the complete device with a rotation plate formeasuring the surface condition;

FIG. 4: the curve of results obtained using the device according to theinvention;

FIG. 5: longitudinal section of the device with rotating mobile support;

FIG. 6: top view of the device with rotating mobile support.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENT

FIG. 1 shows the device 1 according to the invention. This deviceconsists mainly of a reference surface 2 which is the upper face of apattern plate 3 made of diabase, granite or stabilized cast iron whichis itself supported isostatically on a stand 4. The reference plane isground and honed over its entire surface, but a useful area at thecenter is defined to allow for the uncertainty of planeness near theedges due to honing. Therefore, for a total area of 220 mm×220 mm, onlythe central 200 mm×200 mm area is used.

The mobile support 5 is placed on this surface 2. The support consistsof a parallelepiped block with dimensions less than those of thereference block, and the upper part of which overhangs to create twocantilever edges 6, 7. Jacks such as 8 are rigidly attached to theseedges, the force that they exert at their free ends 9 is adjustable, andeach is fitted with a means of rolling such as a ball 10, a thrustbearing or a roller bearing for high pressures. These bearings roll inslides 11, 12 which are parallel to the sides of the block supportingthe cantilever edges. These slides are fixed to two sliding rods 15 and16. The spacing between these two slides is slightly less than the widthof the moving support taken in a direction perpendicular to thecantilever edges such that the support is slightly prestressed withinits elastic limit, thus eliminating play in the direction perpendicularto the displacement direction.

Two counter-slides 111 and 121, located close to the lower face ofslides 11 and 12 prevent the table from tipping during manual sensorinstallation operations.

In order to minimize friction, the mobile support is fitted with ballpistons 18 (FIG. 3) at the sides which are held against the slides by aspring, and can be fixed in position by a pressure screw. The ball canbe replaced by a material with a low coefficient of friction, andgenerally based on polytetrafluoroethylene and its derivatives, with orwithout filler materials, especially the material marketed under thename TURCITE. In this application, TURCITE is used with bronze as afiller material.

The lower part of the mobile support is also fitted with a 19 mm TURCITEplate, but uses graphite as a filler material to improve sliding on thepattern plate.

This arrangement allows better support of the table on its reference andelimination of all lateral clamping, resulting in the possibility ofobtaining perfect alignment of the table with the motor spindle duringassembly; the mobile support 5 is also reamed at its center along adirection parallel to slides 11 and 12, and therefore parallel to thedirection of sliding indicated by the double arrow 20; this reamingallows the free passage of the threaded screw 25 rigidly attached to themotor spindle 26, of the same diameter as the screw fixed to a support27, itself locked on rod 15.

The assembly consisting of the table 5, position adjustable support 27,coupling, reduction gear output, screw and nut makes it possible toaccurately align the motor spindle and the screw to ensure theirconcentricity and avoid lateral play of the screw, which would be thecause of positional uncertainties.

The mobile support is fitted with a self-aligning nut 28 with or withoutplay compensation 28. This nut (hidden in view from FIG. 1) is marketedunder the name GEMTO, and consists of two nuts which are free to take ona relative oscillating movement. This displacement screw has one orseveral triangular threads. It is case-hardened, ground, and has a lowdiameter. The peripheral sliding velocity of the threads is low; this ispossible due to the low tension force resulting from the low coefficientof friction between the mobile support and the pattern plate.

Motor 26 is an electric stepping motor, with a special "HARMONIC DRIVE"reducing gear. The reducing gear consists of a deformable pinion gear,rolling by elastic deformation with no play on an internal tooth gear.Due to this elastic deformation of the pinion, there will always bethree or four teeth in contact in the pinion-wheel pair, leading to asmoothed mean of angular positional errors of the displacement screw;and consequently of positional areas of the mobile support in the X-Yplane.

Rods 15 and 16 are connected at their ends by a yoke 30 and aresupported and guided by four bearings 31 attached to the stand 4.

Yoke 30 is reamed at its center 40 and is also fitted with aself-aligning nut 41 in which a screw 42 is mounted; this assembly isidentical to the one described above. An electric stepping motor 43drives this rod through the intermediate "HARMONIC DRIVE" reduction gear44. This motor 43 is attached directly to stand 4. The directions ofrotation due to this second motorization are shown by the double arrow45. The "HARMONIC DRIVE" type reduction gear is reserved for the top ofthe range, since it can be replaced by a gearwheel reduction gear forbottom of the range product.

FIG. 2 shows an accessory which can be installed on mobile support 5.This is a micrometric forward motion table 50. The table is fixed at 51to the mobile support 5. This table consists of a frame 52 within whichan indicator 53 slides along the Z-axis perpendicular to the X-Y plane.This indicator is driven by the screw nut system 54 of a stepping motorinstalled close to the bottom, to lower the center of gravity of thesystem. This indicator itself supports a cylindric rod 55, at the end ofwhich is attached a second cylindrical rod 56, perpendicular to thefirst. By turning the knurled knob 58, an internal rod 57 displaces thethird cylindrical rod relative to the first, thus locking thecylindrical rod 56 in the bore 59 reamed in cylindrical rods 57 and 55.Rods 55 and 56 are made of a composite material in order to beanti-vibrational.

At its lower end, the vertical rod 56 is fitted with a sensor 60 which,,depending on requirements, can be selected for optical reading, thermalor fluid flow dimensional measurement. This sensor is connected at 61 toa measured results storage and processing unit. The sensor can bereplaced by a micro-engraving tool.

FIG. 3 shows a side view of a device according to the invention as shownin FIG. 1, fitted with a micrometric forward motion table as shown inFIG. 2.

It may be used in addition with a rotatable plate 80, of which part 81is rigidly attached to the table 50 and the other part 82 is rigidlyattached to the mobile support 5. The support stand 4 is made ofcast-iron stabilized at 44° C. after rough machining, and consists of aribbed box which rests on supports under the ribs. Its shape is designedto be made by casting without the use of cores. The legs 91, 92 of atable 90 made of aluminium sections with grooves for the attachment ofparts 93 can be attached to the stand. This table can be fixed or bemade mobile by the addition of a suitable motor drive (not shown).

The part supporting table, like the stand assembly, can also be isolatedfrom external parasitic vibrations by means of conventional dampers.

We will now describe the operation of such a device using the procedureaccording to the invention.

The procedure consists of calibrating jacks 8 and using them to exertindividual forces on slides 15 and 16 such that the sum of these forcesis slightly lower than that exerted by the weight of the mobilestructure and its accessories.

The mobile support/reference surface interface pressure is reduced,making it possible to obtain a regular very high precisiondisplacements, by making the start force similar to the sliding forceand thus limiting geometric positional deviations, taking account ofother precision elements added into the mechanical chain.

The frictionless material also absorbs parasitic vibrations.

In its three-dimensional profile meter measurement application, acontact profile meter contact stylus can explore an arbitrary shapedusing Professor Jean Mignot's large scale procedure.

Measurement characteristics are then related to the quality of thesensor and to the separation power of the contact stylus.

Measurements are made as follows, the purpose being to determinegeometric deviations of shape, rippling and roughness.

Measurements are carried out in space along capture directions X and Y,and along direction Z in depth. The exploration contact stylus istherefore brought into contact with the surface to be checked, withinits measurement range, by table 50 driven by the stepping motor. Themotor is then stopped and the contact stylus stops. One of the twomotors 26 or 43 is then started to apply a translation to the contactstylus along one of the directions X or Y, or the two motors are startedsimultaneously to travel along a parameter-defined, for examplecircular, trajectory (for gasket bearing surfaces). At the same time,deviations detected by the contact stylus are recorded by means of aprocessing and storage unit. When the sensor reaches the limit of itsmeasurement range in Z in a first window, the X or Y displacement isinterrupted, and, depending on the general measurement profile, in otherwords depending on whether the sensor has reached the lower or upperlimit of the measurement range, the table applies a micro displacementupwards or downwards to the contact stylus. The profile is then checkedagain in a second window.

These steps are repeated to cover the entire surface to be checked. Thecomputer processing then connects the successive windows to build up themeasured profile. It should be noted that in this large scale method,the accuracy of measurements is not a function of the Z displacementuncertainty, but only the precision of the sensor.

All these steps are controlled by the processing unit in a known mannerand the result obtained is shown for a specific example in FIG. 4.

The mean accuracy is 0.2 micrometers over a length of 16 mm. The generalslope of the curve is due to a parallelism offset between the plane ofthe surface to be checked and the reference surface, which does notaffect the obtained accuracy. The positioning of the measured profilewithin the system of axes is easily carried out in the processing bycalculating the least squares straight line from the measured profileand then replacing it parallel to the X axis, the measured profile isreplaced in the same manner, and it is also noted that measurement usingthis procedure is independent of the displacement precision along theZ-axis.

For very high precision and micro-roughness, the sensor can be fittedwith a skid resting on a reference surface supported on the part. Thedepth of irregularities in the surface to be explored is then evaluatedrelative to this reference, and the sensor is always moved by X and Ydisplacements and is positioned in the Z direction by the large scaleprocedure. In this case, geometric positional uncertainties of thecontact stylus are always less than 0.05 micrometers.

A three-dimensional image is obtained by processing curves obtainedalong the two axes X and Y put side by side. Therefore, the depth imagecan be anamorphosed differently along X, Y or Z, so that a suitablesubsequent surface treatment can be selected.

The invention is not restricted to the above-mentioned application, butit includes improvements available to men in the profession.

For example, the internal sensor holder rod can be coated internallywith an anti-vibration material, which may also act as counterweight tothe sensor.

There are many applications, since the procedure for obtaining a highprecision and perfectly uniform sliding is applicable to theprofilometer measurement, and also to dimensional measurement, parttransport or displacement of a tool.

Similarly, instead of using a contact stylus, a contact detector couldbe used, mounted on a long travel sensor, in order to consider only theshape of parts.

Still as part of the invention an axial load relief table could be made.

In the same way as for the air cushion in translations, the air supportcan be a hindrance in some applications due to the inherent vibrationsthat it generates due to elasticity of the air film.

This type of axial load relief table is used for the special applicationof replacing high precision air supported vertical towers inenvironments exposed to vibrations.

FIGS. 5 and 6 show an axial load relief table.

The reference surface 100 is coated with an antifriction material at theupper part of the stand 102. Non-outletting bores 103 are reamed in thisframe, in which pistons 104 slide, this forming jacks which, in the mainvariant, can be supplied by air or another fluid, or even replaced byelastic means such as springs.

At their ends, these jacks are fitted with balls 105, to allow the plateitself 106 to rotate, and this plate is also centered on its center line107 inside bore 108 in the stand. There would be advantages in the useof magnetic centering. The load relieving jacks could easily be replacedby a magnetic compensation system. They are laid out at constant angularspacing to ensure isostatic discharge; they may be on concentriccircles.

It is understood that the procedure may be implemented by placing thedevice directly on the parts to be tested, mainly due to the rotation ofthe accessory shown in dashed lines on FIG. 3.

The position of the mobile support can be visually determined within thelimits of its displacements. An X indicator marks the X position of themobile part on a graduated scale.

A Y indicator indicates the Y position of the mobile part on a graduatedscale.

The Z table has a Z identification indicator.

Each mobile part X and Y end of travel is limited by a micro-switchwhich stops rotation of the stepping motor; the same applies in Z.

When the mobile support is in contact with a microswitch, it simplyaccelerates to release it.

The micro-switches do not move with the mobile part, thus avoiding themovement of electric cables.

I claim:
 1. A displacement device having a mobile support slidable on areference surface, said mobile support able to be fit with accessoriesincluding any of a part, sensor and tool, wherein forces are exerted onthe mobile support so that the forces are distributed in a directionfrom a bottom of the support upwards, the displacement devicecomprising:mobile slides having an upper face that support the mobilesupport through a jack and roller devices; and said mobile supporthaving a lower face fit with at least one skid having a low coefficientof friction as well as a self-lubricating, wear resistant, precisionmachinable bearing that contacts the reference surface, and said mobilesupport further including adjustable lateral alignment devices thatcontact the mobile slides.
 2. A device according to claim 1, wherein themobile support is prestressed within an elastic limit between internalfaces of the mobile slides.
 3. A device according to claim 1, whereinthe lateral alignment devices include ball pistons, returned by aspring, and wherein a position of said ball pistons can be locked by apressure screw.
 4. A device according to claim 1, wherein the skid ismade of one of polytetrafluoroethylene, and one of its derivatives,optionally including filler materials.
 5. A device according to claim 1,wherein the mobile support includes a bore fitted with a fixedself-aligning nut along a direction parallel to the mobile slides.
 6. Adevice according to claim 1, wherein the displacement of the mobilesupport is obtained by means of a motor rigidly attached to mobileslides, a screw and a nut.
 7. A device according to claim 6, wherein thescrew has a small diameter and has at least one triangular shaped,case-hardened, quenched and ground thread.
 8. A device according toclaim 1, wherein the reference surface is an upper face of a patternplate block made of a member selected from the group consisting ofdiabase, granite and ground stabilized cast-iron, said block beingisostatically supported on three bearings rigidly attached to a chassis.9. A device according to claim 1, further comprising a table attached tothe mobile support driven by a stepping motor micrometric forwardadvance system, and wherein a Z displacement is perpendicular to a planeof the reference surface.
 10. A device according to claim 9, wherein anindex of the table is fitted with a rod supporting a sensor, said rodbeing oriented perpendicular to the mobile slides.
 11. A deviceaccording to claim 9, wherein a rotatable plate is inserted between themobile support and the micrometric forward advance table.
 12. A deviceaccording to claim 9, wherein the support rod is fitted with a lockingrod with an internal vibration absorbing coating.
 13. A device accordingto claim 1, wherein the sensor is selected from the group consisting ofa thermal, capacitive, fluid flow and optical reading dimensionalmeasurement sensor.
 14. A device according to claim 1, wherein themobile slides are rigidly attached to rods by a translation systemincluding a motor, a reduction gear, a threaded rod, a yoke andself-aligning nut.
 15. A device according to claim 6 or 14, wherein themotor is an electric stepping motor fitted with a gear wheel reductiongear.
 16. A device according to claim 1, wherein the mobile supportcomprises a rotating plate, sliding on skids coated with ananti-friction material, with load relief being supplied through pistonjacks sliding in bores machined in a support stand, and said pistonjacks being fit at their upper end with a ball;wherein the mobilesupport is centered within a bore in the stand.
 17. A displacementprocedure for sliding a measurement device having a mobile supportoptionally including at least one of a part, a sensor and a tool over areference surface, said procedure comprising the steps of exertingdistributed forces from a bottom in an upwards direction on the mobilesupport, wherein a sum of the forces is less than a weight of the mobilesupport to thereby guarantee isostatic load relief.
 18. A procedureaccording to claim 17, wherein the mobile support is rotatable and isslid over an annular surface, the exerting step including exertingforces at constant angular spacings on said mobile platform, whereinsaid forces are exerted over at least one concentric circle.
 19. Aprocedure according to claim 17 or 18, further comprising the step ofapplying the procedure for profiling meter measurements, whereindisplacements are adapted to an operation function to be carried out bymeans of accessories along a Z-axis.
 20. A procedure according to claim19, further comprising measuring three geometric depth deviationparameters, said parameters being shape deviation, rippling androughness and micro-roughness, using a second sensor mounted on amicrometric forward motion table attached to the mobile support.
 21. Aprocedure according to claim 20, further comprising placing items to betested on a support table rigidly fixed to the measurement device sothat measurements may be taken.
 22. A procedure according to claim 20,further comprising performing measurements on large items by placing thedevice directly on the item.